1. Technical Field
The present invention relates to a liquid-crystal display device comprising a polarizer and a retardation film, and in particular to a VA (vertical alignment)-mode liquid-crystal display device.
2. Background Art
It is known that a VA-mode liquid-crystal display device can realize a broad viewing angle, or that is, can have improved display characteristics, as comprising polarizers each disposed on and below a liquid-crystal cell with their absorption axes orthogonal to each other, and having an optically biaxial retardation film disposed between each polarizer and the liquid-crystal cell (for example, Japanese Patent 3330574).
FIG. 6B shows a cross section of a liquid-crystal display device in which the above-mentioned, optically-biaxial retardation films having the same optical anisotropy are disposed on and below the VA-cell; and FIG. 6A shows the polarization state of the light that passes through the constitutive layers, as arrows on a Poincare sphere. In this system, a film of the same type may be disposed on and below the VA cell to enable optical compensation, therefore having the advantage in that the mass-scale production cost is reduced. However, replacing a liquid crystal cell in this system with another, having a different Δnd, always needs to re-optimize retardation in plane Re and retardation along thickness direction Rth of the biaxial retardation films for compensating birefringence of the another liquid crystal cell (for example, as shown in FIGS. 7A and 7B). Variously changing Δnd of the liquid-crystal cell is under investigation for the purpose of power saving and rapid response; and trying to compensate the system employing the such a different liquid crystal cell by two retardation films having the same optical anisotropy, the optimum optical characteristics of the retardation films must be re-planned for each of liquid-crystal cells having a different value of Δnd, and the production line must thereby be re-planned.
On the other hand, a system has been proposed for reducing the light leakage depending on the variation of wavelength of light, or that is, for reducing the color shift in undesirable coloration in blue or red, which is observed in the oblique directions in the black state of liquid-crystal display devices. In the proposed system, used are two retardation films, concretely, an optically-positive monoaxial film (generally A plate) and an optically-negative monoaxial film (generally C plate), having a specific wavelength dispersion characteristics of retardation (for example, Japanese Patent 3648240). FIG. 8B shows a cross section of a liquid-crystal display device of a combination of A plate and C plate; and FIG. 8A shows the polarization state of the light that passes through the constitutive layers, as arrows on a Poincare sphere. In this system, it is unnecessary to change the optical characteristics of the A plate for optical compensation of the liquid-crystal cell of which the Δnd may change variously; and in this, only changing the optical characteristics of the C plate may be enough to satisfy the condition of the changing Δnd of the liquid-crystal cell (for example, as shown in FIGS. 9A and 9B). However, this system requires polarization change with the A plate to the vertical line that passes through the extinction point P, and for attaining the polarization change, Re and Rth of the A plate must be increased. Specifically, the system needs to employ A plate have large Re and Rth; however, it is not easy to produce retardation films having such optical characteristics. It is not also easy to produce on an industrial scale a film satisfying the optical characteristics required for C plate (in a precise sense thereof, its retardation in plane (Re) is zero and its Rth is large). This is because, in industrial-scale continuous production of films, in general, the produced films may have some Re in the machine (or transversal) direction. Such films having some Re are, in a precise sense thereof, optically biaxial films.